A teenage boy accused of fatally stabbing his 3-year-old brother and assaulting his 9-year-old sister in the Phoenix suburb of Youngtown likely will be tried as an adult, authorities said.

The 16-year-old suspect has been booked into the Durango Juvenile Detention Center on suspicion of second-degree murder, aggravated assault and endangerment, said Maricopa County Sheriff Joe Arpaio, whose office is handling the investigation.

The boy's name isn't being released because he's a juvenile.

"This 16-year-old I'm sure will be tried as an adult and will be in our jails," Arpaio said at a news conference Monday.

Arpaio said the teen has no history of mental illness, and there have no prior calls made to the home, located about 25 miles northwest of Phoenix.

Sheriff's officials said Tuesday there were no updates to release in the case, and there still was no word on a possible motive.

Arpaio called it "a vicious, vicious, horrific murder" and said the suspect "was covered in blood" when he was spotted running from the house late Saturday night when sheriff's detectives arrived and tackled the teen.

Neighbors called 911 after seeing the suspect's 17-year-old brother carry his injured sister and a 4-year-old sibling out of the home and seek help.

The 9-year-old girl has been treated and released from a hospital, Arpaio said.

The sheriff wouldn't disclose what detectives found when they executed a search warrant of the Youngtown home and didn't say if a weapon had been recovered.

Neighbors said the family had lived in the home for about six years, and they seldom saw the children outside.

The children's mother was at work at the time of the incident Saturday night.

"We have to get forensic experts to interview the underage witnesses," Arpaio said. "We still have to finish this investigation."